The present invention relates generally to Molecular Beam Epitaxy (MBE) techniques and more particularly to a MBE method of controlling the resistivity of Gallium Nitride films and epitaxial layers.
Gallium Nitride (GaN) is a semiconducting material which has great promise for use in electronic and opto-electronic devices. Semi-insulating to insulating GaN would be particularly useful in construction of high power, high frequency transmitters where it could be utilized to dramatically increase the power output of radar emitters, cell phones, and the like.
One technique for producing GaN films and epitaxial layers is MBE. While MBE is quite useful for the production of GaN epitaxial layers, the GaN epitaxial layers thus produced are generally very conductive. For many optoelectronic devices this is thought of as an advantage, but a shortcoming of this technique is that it doesn""t lend itself to the growth of semi-insulating GaN, described above.
One method for increasing the resistivity of GaN films and epitaxial layers is to dope them with carbon. Carbon doped GaN epitaxial layers exhibit increased resistivity, rendering them suitable for use in the above mentioned high power applications. However, the carbon doping technique used today is difficult to reliably control, since it relies upon ionization of gasses such as methane or propane as the carbon source.
A need exists therefore for a reliable method for controlling the resistivity of GaN films and epitaxial layers. Such a method would be relatively simple and economical to effect yet provide consistent quality, semi-insulating GaN films.
Accordingly, it is a primary object of the present invention to provide a method of controlling the resistivity of GaN overcoming the limitations and disadvantages of the prior art.
Another object of the present invention is to provide a method of controlling the resistivity of GaN utilizing the reliable MBE technique.
Yet another object of the present invention is to provide an MBE method of controlling the resistivity of GaN to produce a semi-insulating GaN film having a preselected desired resistivity.
It is still another object of the present invention to provide a method of controlling the resistivity of GaN utilizing Buckminster Fullerene, C60, as a source of material for carbon doping.
These and other objects of the invention will become apparent as the description of the representative embodiments proceeds.
In accordance with the foregoing principles and objects of the invention, a method of controlling the resistivity of GaN is described. According to the method of the present invention, an MBE system is utilized. MBE is a well known technique that produces consistent high quality results. Pursuant to the MBE process, GaN films and epitaxial layers are grown upon a substrate from raw materials provided from an effusion cell charged with gallium and a source of nitrogen. Advantageously, and according to an important aspect of the present invention, a desired, predetermined GaN film resistivity can be created during the growth process. More specifically, the temperature of the effusion cell containing the C60 is selected from a predetermined range so as to impart the desired resistivity to the GaN film.
It has been determined that the use of C60 in conjunction with varying its effusion cell temperature advantageously provides a resistive GaN film, useful in a wide variety of devices. The range of effusion cell temperatures is relatively narrow, yet the method of the present invention provides for a wide range of film resistivity values. Thus, the method of controlling the resistivity of GaN of the present invention reliably produces semi-insulating to insulating GaN films and epitaxial layers, something not possible by the methods of the prior art.